Cardiac muscle contraction is regulated by Ca2+ through the troponin complex that consists of three protein subunits: troponin C, troponin I, and troponin T (TnT). This research project investigates the role of a novel posttranslational modification of cardiac TnT (cTnT) in myocardial adaptation to energetic crisis. We recently reported a restricted N-terminal proteolysis of cTnT in myocardial ischemia-reperfusion (Zhang et al., Biochemistry 45:11681-94, 2006) and pressure overload (Feng et al., J. Physiol. 586:3537-50, 2008). This modification selectively removes the N-terminal variable region of cTnT but keeps the conserved middle and C-terminal regions intact. Initial studies in transgenic mouse hearts showed that the N-terminal truncated cTnT (cTnT-ND) remains functional in the myofilaments and alters myocardial contractility. This finding has lead to a hypothesis that the proteolytic removal of the N-terminal variable region of cTnT produces a functional state in cardiac muscle thin filaments as acute adaptation to energetic crisis. To test this hypothesis and understand the physiological and pathophysiological significance of cTnT- ND, three Specific Aims are proposed in this research project: Aim I will characterize the effects of the selective deletion of the N-terminal variable region on the function of cTnT. We will examine the interactions of cTnT-ND with other thin filament regulatory proteins and its functional impacts on the Ca2+ activation of myofibril ATPase and the contractility of cardiac muscle. Aim II will study the role of N-terminal truncated cTnT in compensating cardiac function and providing myocardial protection against ischemia-reperfusion injury. The acute and chronic effects of cTnT-ND on heart function and its role in overcoming energetic crisis will be investigated in ex vivo working hearts and in vivo. Aim III will investigate the role of mechanical stretch of the cardiac muscle in inducing the restricted proteolytic cleavage of cTnT N-terminal segment in order to understand the mechanisms that regulates this posttranslational regulation. Using integrated experimental systems, this study will gain new knowledge for the structure-function relationships of TnT and lay a foundation for future development of new treatment of ischemic heart disease and heart failure.